Oncogenesis in pre- and postnatal rat brain by the direct-acting pulse-carcinogen N-ethyl-N-nitrosourea (EtNU) provides a model system for the study of chemical carcinogenesis. A single pulse of EtNU specifically results in a high incidence of neuroectodermal neoplasms; the magnitude of the carcinogenic effect is a function of the developmental stage of the nervous system during which the EtNU-pulse is applied (high-carcinogenicity after exposure during pre- and early postnatal age; decreasing carcinogenicity with increasing age). Fetal rat brain cells transfereed to cell culture after exposure to EtNU in vivo, undergo malignant transformation after a sequence of phenotypic alterations (including, e.g., reduced substrate adherence, morphological transformation, soft agar colony formation, increased growth rate, increased plasminogen activator activity). We intend to compare the probability of neoplastic transformation between different tissues and between phenotypically differing cell populations within a given tissue (brain), as a function of their proliferative and differentiated (developmental) state at the time of exposure to EtNU. In particular we intend to investigate whether the cellular capacity to recognize, eliminate, and repair carcinogen (EtNU)-modified DNA components is connected with the probability of neoplastic transformation; and whether enzymic excision capacity for defined, potentially mutagenic DNA lesions (such as O6-ethyldeoxyguanosine (O6-EtdGuo) varies according to the type and differentiated (developmental) stage of the target cells. These studies are, therefore, concerned with the enzymology of excision repair of ethylated bases in DNA (chromatin), and will compare, with the use of newly developed, highly sensitive immunological detection methods for O6-EtdGuo, the elimination rates of O6-EtdGuo from DNA as well as structurally different fractions of chromatin, in different rat tissues, neural and non-neural cell types, and during different stages of pre-and postnatal development.